During the past years, the interest in radio access technologies for providing services for voice, video and data has increased. There are various telecom technologies used in cellular communications. The most widespread radio access technology for mobile communication is digital cellular. Increased interest is shown in 3G (third generation) systems. 3G systems and, then, even higher bandwidth radio communications introduced by Universal Terrestrial Radio Access (UTRA) standards made applications like surfing the web more easily accessible to millions of users.
Even as new network designs are rolled out by network manufacturers, future systems which provide greater data throughputs to end user devices are under discussion and development. For example, the so-called 3GPP Long Term Evolution (LTE) standardization project is intended to provide a technical basis for radiocommunications in the decades to come.
To increase the transmission rate of the systems and to provide additional diversity against fading on the radio channels, modern wireless communication systems include transceivers that use multi-antennas (often referred to as MIMO systems). The multi-antennas may be distributed to the receiver side, to the transmitter side and/or at both sides as shown in FIG. 1. More specifically, FIG. 1 shows a base station 10 having four antennas 12 and a user terminal 14 having two antennas 12. The number of antennas shown in FIG. 1 is exemplary and not intended to limit the actual number of antennas used at the base station or at the user terminal in the exemplary embodiments to be discussed latter.
The term “base station” is used in the following as a generic term. As it is known, in the Wideband Code Division Multiple Access (WCDMA) architecture, a NodeB may correspond to the base station. In other words, a base station is a possible implementation of the NodeB. However, the NodeB is broader than the conventional base station. The NodeB refers in general to a logical node. A NodeB in WCDMA is handling transmission and reception in one or several cells, as shown for example in FIG. 2. FIG. 2 shows two NodeB 10 and one user terminal 14. The user terminal 14 uses dedicated channels 16 to communicate with the NodeB 10. The two NodeBs 10 are wired to corresponding Radio Network Controllers (RNC) 18. One RNC 18 may control more than one NodeB 10. The RNCs 18 are connected to a Core Network 20. For the LTE architecture, there is a single node, the eNodeB. One possible LTE architecture is shown in FIG. 3, in which the eNodeB 22 may include a physical layer PHY 24, a medium access control MAC 26, a radio link control RLC 28, and a packet data convergence protocol PDCP 30.
Although conventionally the term “base station” is narrower than the NodeB of the WCDMA architecture or the eNodeB of the LTE architecture, the term “base station” is used in the following embodiments as defining the NodeB, eNodeB or other nodes specific for other architectures. Thus, the term “base station” defined and used in the present disclosure is not limited to the conventional base station unit of a network.
LTE was designed to support a number of antennas, for example 1, 2 or 4 antennas. To perform downlink coherent demodulation at the receiver side, the user terminal needs estimates of the downlink channel. One way to enable channel estimation in case of orthogonal frequency-division multiplexing (OFDM) transmission that is used in LTE systems, is to insert known reference symbols into the OFDM time-frequency grid, as shown for example in FIG. 4. As illustrated in FIG. 4, downlink reference symbols 40 are inserted within predetermined OFDM symbols of each slot and with a frequency-domain spacing of, for example, six subcarriers (the downlink reference symbols are not shown to scale in FIG. 4).
To estimate the channel over the entire time-frequency grid, the user terminal may perform interpolation/averaging over multiple reference symbols. Thus, when estimating the channel for a certain resource block (shown for example in FIG. 5), the user terminal may not only use the reference symbols within that resource block but also, in the frequency domain, use neighbor resource blocks, as well as reference symbols of previously received slots/frames. However, the extent to which the user terminal can average over multiple resource blocks in the frequency and/or time domain depends on the channel characteristics. In case of high channel frequency selectivity, the possibility for averaging in the frequency domain is limited. Similarly, the possibility of time-domain averaging, that is the possibility to use reference symbols in previously received slots/subframes, is limited in case of fast channel variations, for example, due to high user terminal velocity.
Generally, to estimate the downlink channel corresponding to each transmit antenna of a transmitting unit, there is one downlink reference signal transmitted from each antenna. In case of two transmit antennas, the reference symbols of the second antenna are frequency multiplexed with the reference symbols of the first antenna, as shown for example in FIG. 6a. In case of four transmit antennas, the reference symbols for the third and fourth antennas are frequency multiplexed within the second OFDM symbol of each slot as shown in FIG. 6b. The reference symbols for antennas three and tour may only be transmitted within one OFDM symbol of each slot. To avoid interference between the reference symbols from various antennas, a resource element carrying a reference symbol for a certain antenna carries no information about the other antennas.
In the case of four transmit antennas, the time-domain reference-symbol density of the third and fourth antennas is reduced compared to the first and second antennas. This arrangement is used to limit the reference-signal overhead in case of four transmit antennas. At the same time, this arrangement has a negative impact on the possibility to track very fast channel variations. This arrangement in LTE systems is maintained because four-antenna spatial multiplexing is applied mainly to scenarios involving low mobility of the user terminal. The reason for retaining the higher reference-symbol density for the first and second antennas in case of four transmit antennas is that it is assumed in LTE systems that these reference signals will be used as part of the initial cell search during which the user terminal has not yet acquired full information about the number of transmit antennas within the cell. Thus, the configuration of the reference signals of the first and second antennas are the same regardless of the number of antennas.
The reference signals and other information initially needed by the user terminal to connect and exchange data with the base station may be achieved via a primary broadcast channel (BCH). In practice, several channels are needed for the user terminal to connect to a certain cell. After acquiring synchronization information by using a synch channel (SCH), the user terminal may decode the primary broadcast channel to obtain at least the minimum system information needed to decode the other channels, including a secondary broadcast channel and/or L1/L2 control channels. Both the synch channel and primary broadcast channel may be transmitted, in one exemplary embodiment, from the base station at regular intervals. The primary broadcast channel has a predetermined number of bits allocated for each frame and each bit may be used to communicate pre-established information about the system. However, it is expensive and difficult to allocate a bit of the primary broadcast channel to include information about the number of antennas used by the base station, information that is need as will be discussed next.
To be able to adapt from low speed scenarios to high speed scenarios, the user terminal will need to support at least two different transmit diversity modes, for two and four antennas, respectively. To decode these modes, accurate channel estimates are needed. As discussed above, common reference symbols are used as training data for computing such estimates. The reference symbols are distributed on all transmit antennas of the base station to allow estimation of the channels of all the antennas. As discussed above, the reference symbols density is lower on transmit antenna three and four to keep the signaling overhead low. This fact reduces the performance of the user terminal at high speed scenarios. Thus, the existing techniques used for high speed scenarios employ transmission modes which utilize only two antennas at the base station, wasting the transmission capabilities of the remaining two antennas.
Thus, it would be desirable to provide methods, devices, systems and software that avoid the afore-described problems and drawbacks.